1. Field of the Invention
The invention relates to transparent substrates, in particular made of a rigid inorganic material such as glass, the said substrates being coated with a stack of thin layers comprising at least one layer having a metallic-type behaviour able to act on solar radiation and/or long-wavelength infrared radiation.
The invention relates more particularly to the use of such substrates to manufacture thermal-insulation and/or solar-protection glazing assemblies. These glazing assemblies are intended to equip both buildings and vehicles, with a view in particular to decreasing the air-conditioning load and/or to reducing excessive overheating caused by the ever growing extent of glazed surfaces in passenger compartments.
2. Discussion of the Background
A known type of multilayer stack for giving the substrates such properties consists of at least one metallic layer, such as a silver layer, which is placed between two coatings of dielectric material of the metal-oxide type. This stack is generally obtained by a succession of depositions carried out using a vacuum technique, such as sputtering, optionally assisted by a magnetic field. Two very thin metal layers may also be provided on either side of the silver layer, the subjacent layer acting as a tie layer for nucleation and the overlayer as a protective or "sacrificial" layer so as to prevent degradation of the silver if the oxide layer which is on top of it is deposited by sputtering in the presence of oxygen.
Stacks of this type, having one or two base layers of silver, are thus known from European Patents EP-0,611,213, EP-0,678,484 and EP-0,638,528.
Currently, there is an increasing demand for these low-emissivity or solar-protection glazing assemblies which have in addition characteristics inherent in the substrates themselves, in particular aesthetic characteristics (so that they can be shape), mechanical properties (so that they are stronger) or safety characteristics (so that they do not cause injury in the event of breakage). This requires the glass substrates to be subjected to heat treatments known per se, of the bending, annealing and toughening type. Laminated-type glazing assemblies intended to be fitted into vehicles, which are nowadays almost all curved and/or toughened, are particularly intended.
It is therefore necessary to adapt the multilayer stack in order to preserve the integrity of the functional layers of the silver-layer type, in particular to prevent their degradation. A first solution consists in significantly increasing the thickness of the thin metal layers, mentioned above, which surround the functional layers: in this way it is ensured that any oxygen likely to diffuse from the ambient atmosphere and/or migrate from the glass substrate at high temperature be "captured" by these metal layers, by oxidizing them, without it reaching the functional layer(s).
This solution is not without drawbacks: since the two metal layers readily oxidize "instead of" the silver layers, they lead in particular to a great increase in the light transmission T.sub.L : it is thus possible to obtain a low-emissivity or solar-protection glazing assembly, which is curved or toughened, having a value of T.sub.L greater than 75 and up to 80%, though this value was much lower before the heat treatment. In particular, reference may be made to Patent Application EP-A-0,506,507 for the description of such a "toughenable" stack with a silver layer placed between a tin layer and a nickel-chrome layer. However, it is clear that the coated substrate before heat treatment was regarded only as a "semi-finished" product and the optical properties frequently made it unusable as such. It was therefore necessary to develop and manufacture, in parallel, two types of multilayer stack, one for the non-curved/non-toughened glazing assemblies and the other for glazing assemblies intended to be toughened or curved, which may complicate matters, in particular in terms of stock and production control.
An improvement proposed in Patent EP-0,718,250 has made it possible to overcome this constraint: the teaching of this document consists in designing a stack of thin layers such that its optical and thermal properties remained virtually unchanged, whether or not the substrate, once coated with the stack, underwent a heat treatment. Such a result is achieved by combining two characteristics:
on the one hand, provided on top of the functional layer(s) is a layer made of a material capable of forming a barrier to oxygen diffusion at high temperature, which material itself does not undergo chemical or structural modification at high temperature which would result in modification in its optical properties. This material may thus be silicon nitride Si.sub.3 N.sub.4 or aluminium nitride AlN; PA1 on the other hand, the functional layer(s) is (are) directly in contact with the subjacent dielectric coating, in particular zinc oxide ZnO.
Although this solution allows the substrate effectively to maintain a T.sub.L level and an appearance in external reflection which, after heat treatment, are quite constant, it is still capable of improvement, in that it has been observed with this type of stack that optical defects, sometimes visible to the naked eye, could appear after heat treatment, these very often being in the form of a speckling of bright spots of the "pinhole" type or having a slightly fuzzy appearance, which is obviously prejudicial in terms of the aesthetic appearance and of productivity since this may lead to an abnormally high scrap rate, most particularly if these glazing assemblies are curved/toughened glazing assemblies, which may or may not be of the laminated type, intended for fitting into vehicles of the motor-vehicle type, in which very strict standards impose a very high optical quality.